The present invention relates to a solid electrolytic capacitor and a method for producing the same. More specifically, the invention relates to a solid electrolytic capacitor with modifications intended for the improvement of the capacitance value and the impedance properties, for the purpose of achieving capacitor downsizing.
Following the recent digitalization and high-frequency modification of electronic appliances, a small-type capacitor with a big capacitance and a low impedance in a high-frequency region has been needed in the field of electric source, in particular.
For such need, a solid electrolytic capacitor has been used, which is produced by assembling a capacitor element by winding together a cathode foil comprising a valve metal such as aluminium and an anode foil comprising a valve metal and having an oxide film prepared on the surface thereof through a separator to prepare a solid electrolyte between these cathode and anode foils. Because such winding-type solid electrolytic capacitor has a characteristic profile of a small type and a large capacitance and also has great impedance properties in a high-frequency region, the capacitor is one of the most suitable capacitors for the need. In such type of solid electrolytic capacitor, additionally, tantalum, niob and titanium other than aluminium are used as the anode material, while metals of the same species as those for the anode material are used as the cathode material.
So as to increase the capacitance of electrolytic capacitor, importantly, the capacitance of the cathode material as well as the capacitance of the anode material should be improved. The capacitance of each electrode of electrolytic capacitor is defined by the type and thickness of an insulation film prepared on the electrode surface and the surface area of the electrode. Given that the dielectric constant of insulation film is xcex5; the thickness of insulation film is t; and the surface area of electrode is A, the capacitance C is expressed by the following equation.
C=xcex5(A/t)
So as to raise the capacitance, as apparently shown by the equation, it is effective to enlarge the electrode surface area, select an insulation film material with a higher dielectric constant and prepare a thinner insulation film.
Attempts have been made conventionally to enlarge the electrode surface area so as to increase the capacitance, because the dielectric constant of insulation film is defined by the electrode material, while the thickness of insulation film is defined by voltage resistance among them. However, just simple use of a large-type electrode to get an enlarged surface area is not preferable, because it leads to the scale up of electrolytic capacitor. Thus, the surface of aluminium foil as the substrate of electrode material has been etching processed traditionally to prepare recesses and protrusions thereon to enlarge the substantial surface area.
Additionally, Japanese Patent Laid-open No. Sho-59-167009 (167009/1984) discloses a cathode material with a metal film prepared on the surface of the substrate by utilizing metal deposition technique, as an alternative of the etching process. According to the technique, film preparation conditions should be selected, to prepare fine recesses and protrusions on the film surface to thereby enlarge the surface area, so that a large capacitance can be recovered. Additionally, metals exerting high dielectric constants in the form of oxides thereof, such as Ti, can elevate the dielectric constant of the insulation film prepared on the surface of the cathode material, leading to a larger capacitance.
Furthermore, Japanese Patent Laid-open No. Hei-3-150825 (150825/1991) previously filed by the present applicant discloses a technique for preparing a deposition layer comprising titanium nitride on the surface of high-purity aluminium used as a cathode electrode by cathode arc deposition process, so as to elevate the capacitance value of the cathode side, in light of the finding that the capacitance of electrolytic capacitor is the composite capacitance of the capacitances of the anode side and the cathode side in serial connection.
[Problems to be Solved]
However, solid electrolytic capacitors using the cathode foils prepared by the conventional techniques described above have the following drawbacks. In other words, the surface of aluminium foil as the substrate of the electrode material in conventional solid electrolytic capacitors is etching processed, so as to elevate the capacitance of the electrolytic capacitors. When etching is processed too excessively, the solubilization of the surface of the aluminium foil concurrently progresses, which adversely blocks the elevation of the surface enlargement ratio. The elevation of the capacitance of electrode material by etching technique was limited.
Additionally, the technique for preparing a deposition layer comprising titanium nitride on the surface of cathode foil has also been problematic. More specifically, manganese dioxide prepared by thermal decomposition of manganese nitrate has mainly been used as the solid electrolyte of conventional solid electrolytic capacitors. During the process of preparing manganese dioxide, however, thermal treatment at 200 to 300xc2x0 C. should be carried out several times. Therefore, oxide film was formed on the surface of the film comprising metal nitride as prepared on the surface of the cathode foil, which caused the reduction of the capacitance of the cathode foil, leading to the reduction of the capacitance of the electrolytic capacitor. Furthermore, ESR reduction was also limited, because manganese dioxide is at a relatively high electric conductivity.
The present invention has been proposed so as to overcome the problems of the conventional techniques. It is a first object of the invention to provide a solid electrolytic capacitor with an improved capacitance value, and a method for producing the same. It is a second object of the invention to provide a solid electrolytic capacitor not only with an improved capacitance value but also with a low impedance at high frequency, and a method for producing the same.
So as to overcome the problems, the present inventors have made investigations about a solid electrolytic capacitor with an improved capacitance value and an improved impedance property, and a method for producing the same. Consequently, the invention has been achieved.
Specifically, it has been found that the capacitance value of a winding-type solid electrolytic capacitor using an organic semiconductor comprising a TCNQ complex salt can be improved, greatly, by preparing a film comprising a metal nitride on the surface of the cathode foil by deposition process.
It has additionally been found that the capacitance value and impedance property of a winding-type solid electrolytic capacitor using an organic semiconductor comprising a TCNQ complex salt can be improved, greatly, by preparing an oxide film on the surface of the cathode foil and additionally preparing a film comprising a metal nitride thereon by deposition process.
First, the inventors have made various investigations about a winding-type solid electrolytic capacitor using an organic semiconductor having been drawing attention recently. Further, N-n-butylisoquinolinium TCNQ complex salt, N-methyl-3-n-propylimidazol TCNQ complex salt, and N-n-alkylisoquinolinium TCNQ complex salt can be used as the organic semiconductor. Herein, TCNQ means 7,7,8,8-tetracyanoquinodimethane. Additionally, these TCNQ complex salts can be prepared by known methods.
Furthermore, the inventors prepared TiN on the surface of cathode foil by deposition process. Using the resulting cathode foil, the inventors prepared a capacitor under the following conditions, to measure the capacitance of the cathode foil alone. It was shown that the capacitance thereof was infinite. This means that TiN prepared on the cathode foil removed a part of the spontaneous oxide film formed on the surface of the cathode foil, so that TiN and the metal of the cathode foil were in continuity.
Additionally, the inventors prepared an oxide film on the surface of the cathode foil at various formation voltages and additionally prepared TiN thereon by deposition process. Using the cathode foil, then, the inventors prepared a capacitor to measure the capacitance of the cathode foil alone. It was shown that the capacitance was infinite. This means that TiN prepared on the oxide film removed a part of the oxide film prepared on the surface of the cathode foil, so that TiN and the metal of the cathode foil were in continuity.
Meanwhile, the capacitance C of electrolytic capacitor is the composite capacitance of the capacitance Ca and the capacitance Cc of the anode side and the cathode side, respectively, in serial connection, which is expressed by the following equations.
1/C=1/Ca+1/Cc
∴C=(Caxc3x97Cc)/(Ca+Cc)=Caxc3x971/(Ca/Cc+1)
As apparently shown by the aforementioned equations, the capacitance C of the capacitor is smaller than the capacitance Ca of the anode side, as long as Cc has a value (the cathode foil has a capacitance). In other words, the capacitance component of the cathode foil is eliminated in case that the capacitance Cc of the cathode foil is infinite, owing to the continuity between the TiN deposited on the surface of the cathode foil and the metal of the cathode foil. Thus, the capacitance C as the composite capacitance of the capacitances of the anode foil and the cathode foil in serial connection, is equal to the capacitance Ca of the anode side. Then, the capacitance C is at maximum.
(Method for Preparing Film Comprising Metal Nitride on Cathode Foil)
As the metal nitride, further, use can be made of TiN, ZrN, TaN, NbN and the like, on the surface of which oxide film is hardly formed. The film to be prepared on the surface of the cathode comprises not only the metal nitride but also other conductive materials with a film-forming potency and with less oxidizability. For example, Ti, Zr, Ta, Nb and the like may be used as such.
Additionally, the method for preparing a film comprising a metal nitride on the cathode comprising a valve metal preferably comprises deposition process, from the respects of the strength of the prepared film, the adhesion thereof to the cathode, and the control of filming conditions. Particularly, cathode arc plasma deposition process is more preferable.
The applicable conditions of the cathode arc plasma deposition process are as follows. Specifically, the electric current is at a value of 80 to 300 A and the voltage is at a value of 15 to 20 V. In case of the metal nitride, further, the cathode arc plasma deposition process is carried out by heating the cathode comprising a valve metal to 200 to 450xc2x0 C. in an atmosphere at the overall pressure inclusive of nitrogen being 1xc3x9710xe2x88x921 to 1xc3x9710xe2x88x924 Torr.
(Method for Preparing Oxide Film on Cathode Foil)
As the method for preparing oxide film on cathode foil, use may be made of general preparation processes for preparing oxide film on anode. Specifically, an oxide film is prepared on the surface of cathode foil by the application of voltage in a formation solution.
As the formation solution of the cathode foil, additionally, use may be made of formation solutions of phosphates, such as ammonium dihydrogen phosphate and diammonium hydrogen phosphate; formation solutions of borates, such as ammonium borate; and formation solutions of adipates, such as ammonium adipate. Among them, ammonium dihydrogen phosphate is preferable. Additionally, the concentration of ammonium dihydrogen phosphate is 0.005 to 3%, preferably.
The formation voltage applied for the formation of the oxide film is preferably 10 V or less. The reason is as follows. When the formation voltage is 10 V or more, the thickness of the oxide film increases, leading to the increase of the thickness of the dielectric film of the cathode foil, so that the capacitance of the cathode foil is reduced, to thereby reduce the composite capacitance of the capacitances of the anode foil and the cathode foil. Further, the formation voltage is preferably 1 V or more, because the effect is reduced below 1 V.
(Method No. 1 for Producing Solid Electrolytic Capacitor)
Continuously, a first method for producing a solid electrolytic capacitor of a winding type is described, where an organic semiconductor comprising a TCNQ complex salt as the electrolyte layer is used. Herein, the first method can achieve the first purpose of the invention.
As the cathode foil, specifically, use is made of a cathode foil, where a TiN film is prepared on the etched aluminium foil by cathode arc plasma deposition process. Further, the conditions of the cathode arc plasma deposition process are as follows. Using a Ti target in nitrogen atmosphere and heating the cathode comprising a valve metal to 200 to 450xc2x0 C., cathode arc plasma deposition is executed at the overall pressure inclusive of nitrogen being 1xc3x9710xe2x88x921 to 1xc3x9710xe2x88x924 Torr and at 80 to 300 A and 15 to 20 V. As the anode foil, further, use can be made of an anode foil with a dielectric film prepared on the surface of the etched aluminium foil by a formation process according to a conventional method. Winding the anode foil together with the cathode foil and a separator, a capacitor element is prepared.
While placing an organic semiconductor comprising a desired TCNQ complex salt in a bottomed cylindrical aluminium case and melting the TCNQ complex salt under heating on a heater heated to about 300xc2x0 C., then, the capacitor element heated to about 250xc2x0 C. is placed in the aluminium case, to impregnate the capacitor element with the melted TCNQ complex salt and immediately immerse the aluminium case in cooling water to cool and solidify the TCNQ complex salt, thereby recovering a desired solid electrolyte layer. Subsequently, the capacitor element is sealed with a resin to prepare a solid electrolytic capacitor.
The time required for the organic semiconductor from the completion of liquefaction to solidification under cooling is set preferably within one minute, more preferably within 15 seconds. As the heating and melting means of such organic semiconductor comprising the TCNQ complex salt as placed in the case, use may be made of the hot plate mode for heating the case on heater block, soldering bath, infrared melting mode, inductive heating mode and the like.
(Method No.2 for Producing Solid Electrolytic Capacitor)
Continuously, a second method for producing a solid electrolytic capacitor of a winding type is described, where an organic semiconductor comprising a TCNQ complex salt is used as the electrolyte layer. Herein, the second method can achieve the second purpose of the invention.
As the cathode foil, specifically, use is made of an etched aluminium foil, which has been formation processed in a 0.005 to 3% aqueous solution of ammonium dihydrogen phosphate at 10 V, and on the surface of which a TiN film has been prepared by cathode arc plasma deposition process. As described above, further, the conditions of the cathode arc plasma deposition process are as follows. Using a Ti target in nitrogen atmosphere and heating the cathode comprising a valve metal to 200 to 450xc2x0 C., cathode arc plasma deposition is executed at the overall pressure inclusive of nitrogen being 1xc3x9710xe2x88x921 to 1xc3x9710xe2x88x924 Torr and at 80 to 300 A and 15 to 20 V. As the anode foil, further, use can be made of an etched aluminium foil, after the surface thereof is treated with a formation process according to conventional methods, to prepare a dielectric film thereon. Winding together the anode foil and the cathode foil through a separator, a capacitor element is prepared.
While placing an organic semiconductor comprising a desired TCNQ complex salt in a bottomed cylindrical aluminium case and melting the TCNQ complex salt under heating on a heater heated to about 300xc2x0 C., then, the capacitor element heated to about 250xc2x0 C. is placed in the aluminium case, to impregnate the capacitor element with the melted TCNQ complex salt and immediately immerse the aluminium case in cooling water to cool and solidify the TCNQ complex salt, thereby recovering a desired solid electrolyte layer. Subsequently, the capacitor element is sealed with a resin to prepare a solid electrolytic capacitor.
The time required for the organic semiconductor from, the completion of liquefaction to solidification under cooling is set preferably within one minute, more preferably within 15 seconds. As the heating and melting means of such organic semiconductor comprising the TCNQ complex salt as placed in the case, use may be made of the hot plate mode for heating the case on heater block, soldering bath, infrared melting mode, inductive heating mode and the like.
Because an oxide film is prepared on the surface of the cathode foil and the metal nitride is prepared thereon in the solid electrolytic capacitor prepared by the second method, as described above, the synergistic effect of the oxide film and the metal nitride may possibly make contributions to the improvement of the chemical stability of the surface of the cathode foil. Additionally, it is shown that the impedance property in a high-frequency region is improved. Owing to the improvement of the stability of the cathode foil, additionally, it is expected that the life duration property can also be improved.
Generally, metal foils processed with etching are used as such cathode foil. However, the use of metal foils with no etching process can never deteriorate the effect of the invention. The inventors additionally confirmed that the use of the cathode foil in accordance with the invention in an electrolytic capacitor using general electrolysis solutions can never yield the maximum capacitance which should be recovered in accordance with the invention, because the capacitance of the cathode foil then is never infinite, which may possibly be due to the formation of an electric bi-layer capacitor in the interface between the electrolyte solution and the cathode foil, which serves as a capacitance component.